An important aspect of muscle development is the ability of motor neurons to exert genetic control over muscle protein synthesis and physiology of post-differentiated muscle. Dependent upon the appropriate innervation, myosin light chains can be switched in muscle. The mechanism by which nerves regulate the synthesis of myosin light chains is thought to be through a combination of impulse activity and release of regulatory factors from the nerve endings. Therefore, two major questions in gene regulation of muscle development prevail. First, how does presumptive muscle cells regulate the transcription of proper contractile gene sequence, e.g., myosin light chains during the early phases of myogenesis? Second, how does innervation regulate myosin light chain gene transcription muscle in the innervated mature muscle? Full length double stranded copy DNA will be made to highly purified myosin light chain mRNAs and the structural gene sequence will be cloned in a bacterial plasmid vector. Cloned DNA sequences will be used to develop more specific gene probes for myosin light chain nucleotide sequences and to determine the locations of various base sequences within the structural genes by restriction endonuclease mapping. Cloned sequences will be used to estimate the number of gene copies in DNA and to quantitate the number of mRNA sequences during muscle differentiation. This is part of a long range goal to eventually isolate the genes which code for myosin light chains and to determine the transcriptional regulatory mechanisms. Part of the long range goal is to utilize specific probes (DNA bound to cellulose) to myosin light chain mRNA to determine the following: the earliest appearance of these gene sequences in nuclei and cytoplasm; the precursor relationship in heavy nuclear mRNA; and the lifetime or stability of these mRNA during myogenesis. The utilization of these probes may provide the tools to unravel the complex mechanisms involved in gene regulation during the induction of muscle protein synthesis in muscle differentiation and during innervation.